Also, a new one that is out called pocket legends. It's beta right now, but can be found for free in the market. It's an mmorpg, which usually isn't my forte, but it's actually really addicting and the graphics are pretty good.

If you like tower defense, try Armored Defense
Has lots of new features like big maps with zoom, towers over railways,...
Graphics are very realistic.
The game is very, very addictive, but a bit hard at the beginning